Subsume PluginLoader functionality under PluginManager

The PluginManager module is now responsible for loading plugins
from the filesystem, deciding what a valid plugin looks like
with regards to the main callback module, and everything else
that previously fell under the PluginLoader module's domain.

The PluginLoader module, meanwhile, has been completely removed
from the Alembic architecture. The PluginManager has taken over
its place in the supervision tree.